How come?
No point and no need. Urban warfare is brutal and has a high casualty rate, and if some estimations of Russia’s primary war goals are correct, they’d already achieved them in the first week of the war - secure a water supply for the Crimea, secure the breakaway provinces, secure the resources off the Crimea. An additional estimation was that they don’t actually want the rest of the country but anything else they can secure with minimal additional risk is a nice bonus.
Keep in mind that one of the unmet goals of the war that got Russia the Crimea was the water source. Most of Crimea’s water comes from Ukraine, and after losing the Crimea they didn’t see a reason to keep supplying them… Not that it justifies the Russian invasion, but you can see why they did it.